1. Technical Field of the Invention
This disclosure relates to a reversible thermosensitive recording medium capable of forming and erasing color developed images by controlling thermal energy using a reversible thermosensitive color developing composition which utilizes the coloring reaction between an electron donative coloring compound and an electron acceptive compound, and to a reversible thermosensitive recording label, a reversible thermosensitive recording member, an image processing apparatus and an image processing method, each using the reversible thermosensitive recording medium.
2. Description of the Related Art
Heretofore, thermosensitive recording media have been widely known that utilize the coloring reaction between an electron donative coloring compound (hereinafter also referred to as a “color coupler or leuco dye”) and an electron acceptive compound (hereinafter also referred to as a “developer”) and, with the development of office automation, the thermosensitive recording media have widely been used as output sheets for facsimiles, word processors, and scientific measurement instruments. Recently, they have also been used as magnetic thermosensitive cards such as prepaid cards or reward cards. It is required for the thermosensitive recording medium, which is put into practical use, to reconsider recycling or reduction of the amount taking account of environmental problems. However, because of irreversible color development, the thermosensitive recording medium can not be repeatedly used by erasing recorded images, and new information can be merely added at the segment where images are not recorded and the area of recordable segment is limited. Therefore, the current measure to overcome this difficulty is to reduce the amount of information to be recorded or to make a new card at the time when no recording area is available. Thus, it is required to develop a reversible thermosensitive recording medium capable of being overwritten as often as desired, against the backdrop of the problems associated with recent garbage and deforestation.
Various reversible thermosensitive recording media have been proposed in response to these requirements. For example, polymer type reversible thermosensitive recording media utilizing a physical change such as transparency or white turbidity are disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-107584 and 04-78573). Also, a dye type reversible thermosensitive recording medium, which utilizes a chemical change, is disclosed. Specifically, there are proposed a reversible thermosensitive recording medium using a combination of gallic acid and phloroglucinol as a developer (see JP-A No. 60-193691), a reversible thermosensitive recording medium using a compound such as phenolphthalein or thymolphthalein as a developer (see JP-A No. 61-237684), reversible thermosensitive recording media wherein a thermosensitive recording layer contains a homogenous compatible material of a color coupler, a developer and a carboxylate ester (JP-A Nos. 62-138556, 62-138568, and 62-140881), a reversible thermosensitive recording medium using an ascorbic acid derivative as a developer (see JP-A No. 63-173684), and a reversible thermosensitive recording medium using a salt of bis(hydroxyphenyl)acetic acid or gallic acid with a higher aliphatic amine as a developer (see JP-A Nos. 02-188293 and 02-188294).
Also, there are developed a reversible thermosensitive color developing composition wherein color development and erasure can be easily conducted under heating and cooling conditions by using an organophosphoric acid compound having a long-chain aliphatic hydrocarbon group, an aliphatic carboxylic acid compound or a phenol compound as a developer and using the compound in combination with a leuco dye as a color coupler and the color developed state and the color erased state can be stably stabilized at normal temperature, and also color development and erasure can be repeated, and a reversible thermosensitive recording medium using the same as a thermosensitive recording layer (see JP-A Nos. 05-124360, 06-210954, and 10-95175).
However, in a conventional reversible thermosensitive recording medium, in case of forming images by heating with a heating element such as thermal head, sticking occurs because of a large frictional force between the heating element and the thermosensitive recording layer, and also periodic irregularity corresponding to dot density of the thermal head is formed on the surface because the surface is likely to be deformed by heat and pressure of a heating element. Therefore, the deformation amount increased while image formation and erasure is repeated, thus making it possible to form clear images.
To solve this problem, there is proposed a method of decreasing a friction coefficient of the surface by providing a protective layer made of a silicone resin or a silicone rubber (see JP-A No. 62-55650). However, because of insufficient adhesion between the protective layer and the thermosensitive recording layer, there arises a problem that peeling is caused by a repeated mechanical action and thus images deteriorate. To improve adhesion, there is proposed a reversible thermosensitive recording medium wherein an intermediate layer made mainly of a resin and a protective layer made mainly of a heat resistant resin are sequentially provided on a thermosensitive recording layer (see JP-A No. 01-133781). According to this proposal, adhesion is improved by the intermediate layer and deformation of the surface of the reversible recording medium is suppressed by the protective layer made of the heat resistant resin. However, in this disposal, when printing and erasing are repeated many times, scratch occurred by sticking or a portion of the protective layer is peeled off and adheres to the thermal head, and accumulation of the peeled material lowers thermal conduction from the thermal head, thus making it difficult to form clear images.
To solve these problems, there is also proposed a reversible thermosensitive recording medium comprising a thermosensitive recording layer and a protective layer, which has not a peak temperature of tan δ (tan δ is a ratio of a dynamic (storage) elastic modulus G′ to a dynamic elastic loss modulus G″, G″/G′) at 250° C. or lower or the corresponding dynamic relaxing phenomenon temperature, formed on the thermosensitive recording layer (see JP-A No. 09-142037). According to this proposal, even if image formation and erasure by a heating element such as thermal head is conducted by the protective layer, periodic occurrence of surface irregularity corresponding to dot density of the thermal head can be considerably suppressed.
However, in case the time of energy to be applied is adjusted to a short-pulse of several milliseconds so as to reduce the time it takes for image processing at the thermal head and applied pressure is increased by a heating element such as thermal head so as to obtain clear images, there arises a problem that cracking occurs on the surface of the protective layer when image formation and erasure is repeated, with speeding up of image processing of an image processing apparatus which conducts image formation and erasure. Also, there arises a problem that the surface state of the protective layer becomes worse as a result of an increase of the amount of cracking and transferability in the image processing apparatus becomes worse, and thus sticking occurs and images deteriorate.
To solve these problems, there is proposed a reversible thermosensitive recording medium comprising a protective layer formed by irradiation with ultraviolet ray at irradiation energy of 150 mJ/cm2 to 1,500 mJ/cm2 so that the protective layer contains an ultraviolet curable resin composition and the ultraviolet curable composition contains an acrylate ester of a bisphenol A diglycidyl ether polymer as an epoxy acrylate having a bisphenol A skeleton, or a dipentaerythritol monohydroxypentaacrylate, and also a tan δ peak temperature of the protective layer, or the corresponding dynamic relaxing phenomenon temperature becomes 155° C. or lower (see JP-A No. 11-334220).
However, the reversible thermosensitive recording media, which have hitherto been proposed, are widely used for applications of magnetic thermosensitive cards such as prepaid cards and reward cards. In these applications, as described above, only an interaction between a thermal head and a reversible thermosensitive recording medium in an image processing apparatus, which conduct image formation and erasure, was discussed and bending of the reversible thermosensitive recording medium by an operator was not assumed at all because of the size of the form and the thickness of the card.
However, recently, the area of applications of the reversible thermosensitive recording medium has quickly been extending to OA equipment such as conference materials for trial printing or single use, and to components control and process control in the factory. In these fields, as compared with a conventional card, display area is widely used within a range from signboard (A6) size to A5 size, A4 size and A3 size. Therefore, since the reversible thermosensitive recording medium is handled in such a manner as in case of a paper, which is not assumed at all in applications of conventional magnetic thermosensitive cards such as prepaid cards and reward cards, a conventional protective layer, whose physical strength with a thermal head is considered to be important, is too hard and cracking occurs in the surface of the medium at the time of handling before repeated printing, and thus images deteriorate. In case images are repeatedly formed and erased, a new problem which has never been anticipated arises, for example, the reversible thermosensitive recording medium curls to the side of the thermosensitive recording surface, and thus it is required to quickly solve these problems.